The pyrolytic degradation mechanism of chestnut shell (CNS) and its blend with waste polystyrene (PS) were investigated. Individual pyrolysis behavior of samples obtained separately was compared with those of the blends using a combined TGA/MS/FT-IR system. To elaborate kinetic analysis and to determine kinetic parameters, distributed activation energy model (DAEM) was used. The average activation energy of co-pyrolytic decomposition reaction was 191.6 kJ/mol, while the activation energy of the pyrolysis of CNS and PS was 175.2 and 208.9 kJ/mol, respectively. Friedman and Flynn-Wall-Ozawa iso-conversional methods were applied and the results were found to be consistent with the models. To express the presence of complex reaction mechanisms and the interactions of the radicals, thermodynamic parameters were also calculated. Finally, the pathways for main volatiles were established, and their relationship with the pyrolytic degradation was suggested.